The present invention relates generally to the treatment of matter by interaction with plasma and, in particular, to a method for the treatment of waste material via discrete electrical discharges.
The relatively high energy levels available from plasma have recognized value in the processing of many particulate materials. Typical known methods of generating plasma have relied on the conversion of electric current into high temperature effluents of one kind or another which, by virtue of their high temperature and enthalpy, interact with the feedstocks, bringing about a higher rate of reaction than would take place at a low temperature. Most conventional methods of producing low temperature plasma, plasma torches ("plasmatrons"), for example, have not proven industrially useful for the treatment of tonnage quantities of particulate matter, in themselves.
The difficulties encountered in the treatment of particulate materials with low temperature plasmas are discussed in U.S. Pat. No. 4,361,441 issued Nov. 30, 1982 in the name of Jozef K. Tylko, the inventor herein, which patent is hereby incorporated by reference. The expression "low temperature plasma" as defined in the incorporated patent is an arbitrary reference to plasmas having an ion temperature below 100,000.degree. K. That convention is retained herein. Further, the term "particulate matter," as used herein, includes solid particles as well as fluids (gases or liquids), and combinations or mixtures thereof.
As described in the incorporated patent, the industrial application of low temperature plasma technology has developed along two different routes. The first route (in which the volume of the arc discharge was not used for entrainment of feed stocks) chiefly utilized the point of impingement of the arc at the anode and behaved in this respect very much like electric arc furnaces. The second route, more relevant to the present invention, aims at the treatment of particles in the whole volume of the plasma--treatment "in flight." For this purpose, it was required to expand the plasma and this involves increasing its original volume. This is the development of Jozef K. Tylko, the inventor herein.
Two distinct methods for plasma expansion are discussed in the incorporated patent. It should be noted that both of these methods involve the establishment and maintenance of an expanded plasma volume. The more relevant of these methods to an understanding of the present invention is initially disclosed in British Patent Nos. 1,390,351-3. In this method, a plasma torch acting as a cathode was made to orbit in a circular path and at a small angle with the vertical, projecting the arc to a downstream annular electrode. A truncated conical plasma region was defined by the orbiting arc discharge. This method served well as a laboratory plasma furnace for studying many reactions. However, mechanical limitations resulting from the need to orbit the torch severely limited the industrial utility of this method.
The method of the incorporated patent addressed many of the limitations of the orbiting torch technology discussed above. In this methodology, an arc discharge was generated between stationary electrode structures, at least one of those structures being annular. As indicated in the incorporated patent, a primary requirement of the system was the establishment and maintenance of an expanded plasma within the reaction zone between the electrode structures. Early versions of the methodology of the incorporated patent also required utilization of a plasmatron. Later versions utilized a simple electrode structure as the central cathode while maintaining an annular ring structure for the anode, the annular anode being formed by multiple anode segments. Such a system is discussed in a thesis prepared by Steven Anthony Vrchota in partial fulfillment of the requirements of a degree of Masters of Science at the University of Minnesota dated April, 1991 and entitled Use of the Sustained Shock Wave Reactor for the Recovery of Metals From Electric Arc Furnace Dust.
The method of the incorporated patent allowed a heavier loading of matter to be treated while taking advantage of an apparently anomalous behavior of at least some matter in such plasmas. It was concluded that the presence of a dense suspension of particles in the plasma zone increased the effective energy flux causing rapid plasma-solid interactions. It was noted that these interactions need not be purely thermal and, indeed, that the method of the incorporated patent utilized these non-thermal (anomalous) phenomena.
As indicated above, the difficulty in these prior art methods is maintenance of the plasma-forming arc discharge. Generally, as the quantity of matter introduced into the expanded plasma increased, the likelihood of extinction of the arc increased. Extinction of the arc stops the processing, with many attendant problems. This difficulty was addressed in the incorporated patent by the utilization of a rapidly fluctuating potential difference between the cathode and the anode electrode structures while that discharge was circulating about the periphery of the annular electrode structure (anode). However, maintaining the plasma at industrially significant feedstock loading remained a problem. A pattern of least energy configuration would be established. Also, significant loading volumes remained a problem in themselves, in that not all particles were treated.
In summary, the method of the incorporated patent resolved the mechanical limitations in the prior art by orbiting the arc by non-mechanical means whereby rapid rates of "rotation" far greater than those possible by mechanical means were attained. Additionally, pulsation of the arc by rapid changes in the applied power established conditions which improved the ability to maintain the arc discharge while also utilizing non-thermal phenomena which enhance the plasma action on the matter being treated. However, the perceived need to maintain the arc discharge orbiting around the annular electrode structure significantly limited the industrial utility of the method. Also, significant loading volumes remained a problem. During operation, a pattern of least energy configuration was established. As a result, particles were not uniformly treated.